With X-ray Computed Tomography (CT) apparatuses, a technique called photon counting CT is known by which an image is rendered by counting the quantity of X-ray photons. During a photon counting CT procedure, an X-ray detector of a photon counting type (hereinafter, “photon counting X-ray detector”) is used. The photon counting X-ray detector is configured to measure intensities of X-rays by counting the quantity of X-ray photons that have become incident thereto. Further, the photon counting X-ray detector is configured to measure the energy level of each of the X-ray photons, by using the notion that, when each X-ray photon is converted into an electric charge, the amount of electric charge occurring corresponds to the energy which the X-ray photon has. Accordingly, by performing the photon counting CT procedure, it is possible to obtain an energy spectrum indicating a distribution of quantities of photons corresponding to the energy levels.